NO. I THE INSECT HEAD SNODGRASS 47 



region before the first segment. The latter Chaudonneret calls the 

 "preantennular" segment, the ganglia of which become the proto- 

 cerebrum, and the appendages the eye stalks of Crustacea. The 

 second segment is that of the first antennae, the third is the second 

 antennal segment, the ganglia of which become the tritocerebrum. 

 The fourth segment is the segment of the superlinguae, the next 

 three those of the mandibles, maxillae, and labium, Chaudonneret 

 thus, by reviving the long discredited superlingual segment and 

 omitting the preantennal segment of other writers, makes out seven 

 segments in the adult head. 



On the other hand, from a comparative study of the internal or- 

 ganization of the brain in the Polychaeta, Onychophora, and Arthrop- 

 oda, Holmgren (1916) and Hanstrom (1928) have very reason- 

 ably argued that the entire preoral head lobe of the arthropod 

 embryo represents the annelid prostomium. First it is to be noted that 

 both the prostomium of the worm and the head lobe of the arthro- 

 pod are preoral, and show no external evidence of segmentation. 

 Second, the part of the arthropod brain formed inside the cephalic 

 lobe shows a striking resemblance to the prostomial archicerebrum 

 of the polychaetes. 



The brain of the Polychaeta innervates the anterior tentacles, the 

 eyes, and the prostomial appendages known as the palps. The brain 

 centers of the palpal nerves lie behind the optic centers and are 

 closely associated with the corpora pedunculata. In some families 

 the ganglia of the first postoral segment, from which arises the 

 stomatogastric system, are united with the brain. 



The brain of Onychophora consists of the primitive prostomial 

 brain and the secondarily added first postoral ganglia. The tentacles 

 of the annelids are absent in the Onychophora, but the forebrain in- 

 nervates the eyes and the antennae. The antennal commissure lies 

 behind the optic centers and the antennal nerve centers are associated 

 with the corpora pedunculata just as are the palpal centers in the 

 polychaete. The onychophoran antennae thus would appear to repre- 

 sent the polychaete palpi. The onychophoran brain, as that of the 

 arthropods, contains a central body. The hind brain innervates the 

 feeding organs known as the "jaws," which thus correspond with 

 the chelicerae or second antennae of the arthropods. It gives origin 

 to the stomatogastric nerves, and its component ganglia are connected 

 by a suboesophageal commissure. 



The internal structure of the arthropod brain closely resembles 

 that of the onychophoran brain, except that the ocular and antennal 



